Conventionally, class D audio amplifiers have the benefit of high power efficiency, but such amplifiers can also have a drawback in terms of electromagnetic interference (EMI), which can interfere with nearby wireless receivers, violate Federal Communication Commission (FCC) emission limits, or any combination thereof. Audio Class D amplifiers often switch at a frame rate of a few hundred kHz, and common mode energy at a carrier frequency and its harmonics can fall directly in the amplitude modulated (AM) radio frequency band, interfering with nearby AM receivers.
FIG. 1 illustrates a graph 100 of a “BD modulation” employed by many class D amplifiers. Class D type BD modulation varies pulse widths of two pulse waves that are time-aligned and often nominally centered within a pulse-width modulated (PWM) frame, which has a frame width (T). For positive input signals, the pulse width PWM B signal 102 that drives the high side of the bridged output (typically referred to as a P or B pulse) is increased (such as by a delta (Δ)) while the pulse width of PWM D signal 104 that drives the low side of the bridged output (typically referred to as an N or D pulse) is decreased (such as by the delta (Δ)). For negative PWM input signals, a width of the PWM D (or N) signal 104 is increased while the width of the PWM B (or D) signal 102 is decreased, resulting two similar but negative differential pulses. Differentially, this is an efficient arrangement since there is no wasted differential energy.
In this example, a differential mode signal 106 includes pulses that are nominally centered at ±T/4, where T is the width of the PWM frame and the reference time position T=0 represents the center of the frame. The differential mode signal 106 is applied across the load (such as a filter in cascade with a speaker). The carrier frequency of the differential mode signal 106 is at twice the PWM frame rate. However, the common mode signal 108 has a peak energy that is nominally centered at the PWM frame rate. Carrier energy of the common mode signal 108 can interfere with nearby circuitry or radio receivers.
FIG. 2 illustrates a graph of a resulting differential mode power spectrum 200 at the output of an associated H-bridge circuit. As shown, the graph 200 illustrates the differential mode component at twice the frame rate in the frequency domain, where the frame rate is 960 kHz.
FIG. 3 illustrates a graph of a resulting common mode power spectrum 300 at the output of an associated H-bridge circuit, showing a common mode component at the frame rate of 960 kHz. The strong common mode component created at the PWM frame rate, as illustrated by the common mode power spectrum 300, can interfere with nearby radio receivers. Given that practical switching frequencies for audio applications range from approximately 200 kHz to 1000 kHz and that the AM band ranges from 520 kHz to 1710 kHz, there is a problem with radiated interference of the common mode carrier and its harmonics interfering with reception of an AM receiver in close proximity to or within the same system. Therefore, it is desirable to suppress the common mode carrier of a class-BD double sided symmetrical modulated signal with little or no compromise in the differential mode performance.